Transmission line directional awareness

ABSTRACT

An apparatus and method for coupling a charging station to a power line segment that is terminated at a first end by a charging terminal are disclosed. The apparatus includes multiple taps coupled to the power line segment and circuitry coupled to the charging station and coupled to the multiple taps. The circuitry is configured to differentiate between communication signals propagating on the power line segment in the direction from the first end to a second end of the power line segment and communication signals propagating on the power line segment in the direction from the second end to the first end based at least in part on multiple measurements of respective phase shifts associated with different portions of a communication signal, each portion received over at least a first tap and a second tap.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Application Ser. No.61/219,661, filed on Jun. 23, 2009, incorporated herein by reference.This application is a continuation-in-part of U.S. application Ser. No.12/421,452, filed on Apr. 9, 2009, which claims priority to U.S.Application Ser. No. 61/043,581, filed on Apr. 9, 2008, each of which isincorporated herein by reference.

TECHNICAL FIELD

The invention relates to directional awareness for detectingelectromagnetic signals propagating on a transmission line.

BACKGROUND

Power Line Communications (PLC) is a rapidly growing market. PLC isattractive because it uses existing power lines that are ubiquitous inhomes and businesses around the world. PLC products have proven to bevery successful for in-home data distribution. Other market segmentssuch as Smart Grid (power utilities controlling power distributioninfrastructure and major electrical loads) and Access BPL (use of powerlines to provide high speed internet access to customers not served bycable or DSL) call for broadband data to be transmitted to homes andbusinesses over outdoor power lines.

SUMMARY

In one aspect, in general, an apparatus is described for coupling acharging station to a power line segment that is terminated at a firstend by a charging terminal. The apparatus includes multiple taps coupledto the power line segment; and circuitry coupled to the charging stationand coupled to the multiple taps, with the circuitry being configured todifferentiate between communication signals propagating on the powerline segment in the direction from the first end to a second end of thepower line segment and communication signals propagating on the powerline segment in the direction from the second end to the first end basedat least in part on multiple measurements of respective phase shiftsassociated with different portions of a communication signal, eachportion received over at least a first tap and a second tap.

Aspects can include one or more of the following features.

The communication signal comprises multiple subcarriers with differentfrequencies, and the respective phase shifts associated with differentportions of the communication signal comprise phase shifts associatedwith different subcarriers.

Each of the multiple measurements of the respective phase shiftscomprises determining a phase for a subcarrier received over the firsttap relative to a phase for the subcarrier with the same frequencyreceived over the second tap.

The communication signal comprises multiple symbols occurringsequentially in time, and the respective phase shifts associated withdifferent portions of a communication signal comprise at least somephase shifts associated with different subcarriers and at least somephase shifts associated with different symbols.

The communication signal comprises multiple symbols occurringsequentially in time, and the respective phase shifts associated withdifferent portions of a communication signal comprise phase shiftsassociated with different symbols.

The communication signal comprises multiple subcarriers with differentfrequencies, and each of the multiple measurements of the respectivephase shifts comprises determining a phase for a subcarrier receivedover the first tap relative to a phase for the subcarrier with the samefrequency received over the second tap.

The circuitry comprises: a first phase detector connected to the firsttap; a second phase detector connected to a second tap; and a comparatorwith inputs connected to an output of the first phase detector and anoutput of the second phase detector.

The circuitry comprises: a first adder connected to the first tap via afirst transmission line and connected to the second tap via a secondtransmission line that is longer than the first transmission line; asecond adder connected to the second tap via a third transmission lineand connected to the first tap via a fourth transmission line that islonger than the third transmission line; and a comparator with inputsconnected to an output of the first adder and an output of the secondadder.

The charging station is a vehicle charging station that provides powerto a vehicle connected to the charging terminal.

The charging station is configured to determine billing information fromreceived communication signals propagating on the power line segment inthe direction from the first end to the second end.

The charging station is configured to determine billing information fromcommunication signals transmitted by a vehicle connected to the chargingterminal.

A first tap on the power line segment is spaced a distance D from asecond tap on the power line segment.

The distance D is less than about one tenth of the wavelength of atleast one of a predetermined number of the highest of multiple carrierfrequencies of the communication signals.

The distance D is less than about one tenth of the wavelength of thelowest of the predetermined number of the highest of multiple carrierfrequencies of the communication signals.

The circuitry comprises: a signal processing unit in communication witheach of the taps, wherein the signal processing unit is configured tomeasure each phase shift by determining a phase for at least a portionof a signal received over at least a first of the taps relative to aphase for a corresponding portion of the signal received over at least asecond of the taps.

The communication signal comprises multiple subcarriers, and the signalprocessing unit is configured to measure each phase shift by determininga phase for each subcarrier received over at least a first of the tapsrelative to a phase for a corresponding subcarrier received over atleast a second of the taps.

The corresponding subcarrier received over the second of the taps hasthe same frequency as the subcarrier received over the first of thetaps.

The multiple subcarriers comprise fewer than all subcarriers on whichthe signal is modulated.

The multiple subcarriers comprise a predetermined number of subcarrierswith the highest frequencies.

The signal processing unit is configured to apply a weight to eachmeasured phase shift according to a signal-to-noise ratio associatedwith the subcarrier for which the phase shift is measured.

In another aspect, in general, a method is described for coupling acharging station to a power line segment that is terminated at a firstend by a charging terminal. The method includes sensing a communicationsignal propagating on the power line and being coupled through multipletaps connected to the power line segment; and determining if thecommunication signal is propagating on the power line segment in thedirection from the first end to a second end of the power line segmentor in the direction from the second end to the first end based at leastin part on multiple measurements of respective phase shifts associatedwith different portions of the communication signal, each portionreceived over at least a first tap and a second tap.

The communication signal comprises multiple subcarriers with differentfrequencies, and the respective phase shifts associated with differentportions of the communication signal comprise phase shifts associatedwith different subcarriers.

Each of the multiple measurements of the respective phase shiftscomprises determining a phase for a subcarrier received over the firsttap relative to a phase for the subcarrier with the same frequencyreceived over the second tap.

The communication signal comprises multiple symbols occurringsequentially in time, and the respective phase shifts associated withdifferent portions of a communication signal comprise at least somephase shifts associated with different subcarriers and at least somephase shifts associated with different symbols.

The communication signal comprises multiple symbols occurringsequentially in time, and the respective phase shifts associated withdifferent portions of a communication signal comprise phase shiftsassociated with different symbols.

The communication signal comprises multiple subcarriers with differentfrequencies, and each of the multiple measurements of the respectivephase shifts comprises determining a phase for a subcarrier receivedover the first tap relative to a phase for the subcarrier with the samefrequency received over the second tap.

The charging station is a vehicle charging station that provides powerto a vehicle connected to the charging terminal.

The communication signal includes billing information transmitted from avehicle connected to the charging terminal.

The second end is connected to a power distribution network.

The communication signal includes billing information transmitted from avehicle connected to the charging terminal.

The communication signal includes billing information transmitted from aserver connected the power distribution network.

Among the many advantages of the invention (some of which may beachieved only in some of its various aspects and implementations) arethe following.

Directional awareness techniques facilitate the use of PLC for electricor hybrid vehicle charging stations with integrated billing by enablingthe charging station to uniquely identify signals transmitted from thevehicle connected to a terminal associated with the charging station. Asubstantial deployment of electric vehicles calls for the development ofpower charging infrastructure to service those vehicles. Chargingstations in parking lots and other public places have the potential toconveniently meet that demand.

Billing for power drawn at charging stations can be efficientlyimplemented by transmitting billing signals over the power distributionnetwork itself using PLC. PLC equipment can be used to communicateinformation between an electric vehicle (EVs) and a charging station. APLC based billing method for an electric or hybrid vehicle chargingsystem would have a number of advantages over alternative automatedbilling methods. For example, a PLC based billing method saves the costof building out a parallel communications network to support billing andcan potentially be more secure than alternative billing methods, such asthose that rely on wireless communications links.

One aspect of automated billing at vehicle charging stations includescollecting vehicle or customer identification or billing authorizationinformation and matching that information with a measurement of energyconsumed at a particular charging terminal. For isolated chargingterminals, there is no potential confusion since there is only onecharging station that can be expected to receive the billing informationtransmitted by a vehicle or customer. In the case of clusters ofcharging stations that collect billing information over a sharedcommunications medium, there may be potential confusion. In manyinstances, such as public parking lots or apartment buildings, many EVsand charging stations will be present. Electric power lines are a sharedmedium. Signals from an EV will often reach well beyond the chargingstation to which it is physically connected. In order to ensure accuratebilling information, it is essential that a charging station is able touniquely identify those signals transmitted from the vehicle to which itis attached.

An exemplary PLC based solution to the bill matching problem is to havevehicles connect to a power supply through a power line segment that ismonitored by a charging station with PLC capabilities and the ability todetect in which direction a PLC message is propagating on the power linesegment. The ability to detect the direction of propagation for a signalon a transmission line, such as a power line can be called directionalawareness. A charging station with directional awareness is able todistinguish messages sent over the power line by a vehicle connected toits charging terminal from PLC messages sent by other nodes connected tothe same power line serving a cluster of charging stations. Byidentifying messages from vehicles at its terminal, the charging stationmatches that information to energy consumption that it measures and canrelay the matched information as needed to other nodes in the PLCnetwork to complete a billing transaction.

Directional awareness can be achieved using multiple measurements ofrespective phase shifts associated with different portions of acommunication signal. The diversity of the multiple measurements canlead to improved sensitivity, especially with small tap spacing and lowsignal-to-noise-ratio. For example, using multiple subcarriers canprovide frequency diversity and using multiple symbols can provide timediversity, as described in more detail below.

Other features and advantages of the invention will be found in thedetailed description, drawings, and claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of local area power distribution networksupporting multiple vehicle charging stations.

FIGS. 2A and 2B are block diagrams of a directional awareness sensor.

FIG. 3 is a schematic diagram of a power line directional coupler.

FIG. 4 is a block diagram of a communication system implementing an OFDMmodulation scheme.

FIG. 5 is a plot of the frequency response of the power line directionalcoupler system in the forward transmission direction without pre-scalingof the subcarrier signals.

FIG. 6 is a plot of the frequency response of the power line directionalcoupler system in the forward transmission direction with pre-scaling ofthe subcarrier signals.

DETAILED DESCRIPTION

There are a great many possible implementations of the invention, toomany to describe herein. Some possible implementations that arepresently preferred are described below. It cannot be emphasized toostrongly, however, that these are descriptions of implementations of theinvention, and not descriptions of the invention, which is not limitedto the detailed implementations described in this section but isdescribed in broader terms in the claims.

A local area power distribution network 100 is depicted in FIG. 1. Thenetwork includes a power line 110, which connects the nodes in the localarea network to each other and a power source 105. The power source 105supplies power, for example, as a 120 Volt Alternating Current (120 VAC)waveform. For example, the power source 105 may include a step downtransformer that couples power to the local area distribution networkfrom a high voltage transmission line of a larger power grid. The powerline 110 includes one or more power line segments 111, 113, 115, 117,and 119. Each power line segment connects at one end to the rest of thepower line 110 and terminates at the other end at a charging terminal130, 132, 134, 136, or 138. A charging terminal is an interface, such aplug, that is used to physically connect the power line to a device tobe charged, such as an electric vehicle 140, 142, 144, 146, or 148. Eachpower line segment also passes through and connects to a chargingstation 120, 122, 124, 126, or 128 via two or more taps. The local areapower distribution network 100 may also include a local authenticationserver 150 that connects to the power line 110 and is configured as PLCnode capable of processing billing transactions via messages transmittedand received over the power line 110.

Each charging station (e.g., 120) is configured to measure the powersupplied at its associated charging terminal (e.g., 130) through thepower line segment (e.g., 111) to which it is attached. For example, acharging station may include a current transformer that measures currenton the power line segment indicating the energy consumed during acharging session. The charging station is also a PLC node withdirectional awareness. The charging station includes PLC circuitry thatis able to detect when a communication signal is being transmitted overthe power line segment (e.g., by detecting a preamble and header of adata frame). When a vehicle (e.g., 140), or other device to be charged,is connected to the charging terminal, it may transmit a signal on thepower line segment carrying a message that includes billing information(e.g., information identifying a vehicle or customer for the purpose ofbilling). This message is received by the charging station 120, whichassociates the billing information with energy consumption during adetermined charging session via the power line segment 111. Theassociated billing information may be forwarded by the charging station120 to an authentication server 150 for further processing andauthentication. The charging station 120 may also receive billinginformation from the server 150. The determined charging session maybegin after the authentication and may end, for example, when thecharging terminal is disconnected, or when the measured current dropsbelow a predetermined threshold, or after a predetermined interaction(e.g., a button press). Once charging session is complete, the measuredenergy consumption information along with any required billinginformation is transmitted to the local authentication server or anotherPLC node reachable via the power line 110 with the ultimate destinationbeing a device configured to complete a billing transaction. Aconfirmation message, such as an electronic receipt may be received bythe charging station 120 and forwarded to the charged vehicle 140.

Charging stations are able to differentiate between signals propagatingin different directions on the power line segment and are therefore ableto ignore billing information transmitted on the power line by vehiclesconnected to other charging terminals. In one example, a second vehicle142 is connected to the local area power distribution network 100 via asecond charging terminal 132. When the second vehicle 142 transmits itsmessage with billing information on the power line 110 (from the powerline segment 113), the signal bearing that message could propagate toall charging terminals in the network 100. However, only the chargingstation 122 on its power line segment 113 will process this billinginformation which is associated with power metered by the chargingstation 122. All other charging stations 120, 124, 126, and 128 willignore the message because the direction of signal arrival will be fromthe network end of their respective power line segments 111, 115, 117,and 119 (as opposed to the charging terminal end).

In some implementations, the charging terminals are configured so thatvehicles (e.g., 140) present a matched load at the charging terminal(e.g., 130), so reflections due to impedance mismatch at the terminalare minimized and the sensitivity of directional awareness in thecharging station (e.g., 120) is maximized. When no vehicle is connectedto the charging terminal, an impedance mismatch may occur. In this casethe charging station may detect the absence of a connected vehicle atits terminal and use that information to reject or ignore billinginformation messages received while no vehicle is connected.Alternatively, there may be situations in which a vehicle is connected,but is not actively charging. The terminal to which the vehicle isconnected can include circuitry to control whether a vehicle is in acharging state.

FIG. 2A shows a block diagram of an exemplary directional awarenesssystem 200 that may be employed in a charging station. The system 200includes taps to couple to a power line segment 210 at two positions,Tap A 214 and Tap B 216, separated by a distance d. Tap A is connectedby a transmission line 222 (e.g., any arrangement of conductors such asa cable or a pair of wires) to one input of a first adder 232 that maybe positioned close to Tap A. A second input of the first adder 232 isconnected to Tap B by a transmission line 226 that is longer than thefirst transmission line 222 by a length of approximately d. Tap B isconnected an input of a second adder 234 by a third transmission line224. A second input of the second adder 234 is connected to Tap A byfourth transmission line 228 that is longer than the third transmissionline 224 by a length of approximately d. The adders 232 and 234 each addthe signals detected at their inputs and produce an output signal, 242and 244 respectively, that is proportional to the amplitude of theresulting AC waveform. For example, the adders 232 and 234 may rectifyand lowpass filter the summed AC waveforms to produce their outputsignals. The adder output signals 242 and 244 are then passed to theinputs of a signal processing unit such as a comparator 250. The output260 of the comparator 250 may be monitored to determine the direction inwhich the signal on the power line segment 210 is propagating. When theoutput has positive sign (i.e., signal 242>signal 244) the signal on thepower line segment is propagating in the direction from Tap B to Tap A.When the output has negative sign (i.e., signal 242<signal 244) thesignal on the power line segment is propagating in the direction fromTap A to Tap B.

Alternatively, other implementations of the directional awareness systemcan compare the phase shift at Tap A relative to the phase shift at TapB in other ways. For example, the system can directly measure the phaseshifts at Tap A and Tap B at the same time and compare the resultingphase shift measurements. When the phase shift at Tap B lags the phasesift at Tap A by an amount corresponding to the distance d, the signalon the power line segment is propagating in the direction from Tap A toTap B; and when the phase shift at Tap A lags the phase sift at Tap B byan amount corresponding to the distance d, the signal on the power linesegment is propagating in the direction from Tap B to Tap A.

FIG. 2B shows a block diagram of an exemplary directional awarenesssystem 270 that compares relative measured phase shifts between thetaps. The system 270 also includes the taps to couple to the power linesegment 210 at two positions, Tap A 214 and Tap B 216, separated by adistance d. Tap A is connected by the transmission line 222 to an inputof a receiver 272 that may be positioned close to Tap A. Tap B isconnected by the transmission line 224 to an input of a receiver 274that may be positioned close to Tap B. The receivers 272 and 274 eachmeasure respective phases of the signals detected at their inputs andproduce an output signals, 276 and 278 respectively. The output signalsinclude information about the phases of one or more subcarriers detectedby the receivers at their respective taps. The output signals 276 and278 may also bear other information about the subcarriers, such as thefrequency of the subcarrier and the signal to noise ratio (SNR) detectedfor each subcarrier. The output signals 276 and 278 are then passed tothe inputs of a signal processing unit 280. The signal processing unitcompares the relative phase from one or more subcarrier received fromTap A to the phase of corresponding subcarriers received at Tap B toestimate the relative phase shift between the taps experienced by thesignal propagating on the power line. The output 290 of the signalprocessing unit 280 may be monitored to determine the direction in whichthe signal on the power line segment 210 is propagating.

In the implementation of FIG. 2A, when the distance d is approximatelyone quarter of a wavelength of a given subcarrier frequency in thesignal on the power line segment 210, the waveforms received at one ofthe adders add constructively, and the waveforms received at the otheradder add destructively. For example, for a signal propagating from TapB to Tap A, the signals arriving at the inputs of adder 232 havetraveled approximately equal lengths and add constructively. Whereas thesignal arriving at adder 234 over the transmission line 228 has traveledapproximately 2 d further than the signal arriving at adder 234 over thetransmission line 224. Thus, when d is a quarter wavelength, one signalis a half wavelength out of phase, equivalent to a phase shift of it (or180 degrees). However, even if the distance d is less than a quarterwavelength, it is still possible to distinguish the different amounts ofconstructive and/or destructive interference that occur between adder232 and adder 234. For example, for a distance d of one tenth of awavelength, the output 260 is about 20% of its value at a distance d ofa quarter wavelength. Depending on the noise or distortion on the powerline a distance significantly smaller than a quarter wavelength maysuffice to provide adequate directional awareness for determiningwhether or not a message is from a local vehicle (e.g., d between about0.25 to 0.1 of a wavelength, or d between about 0.25 to 0.05 of awavelength). Additionally, for communication schemes that use multiplesubcarrier frequencies (e.g., OFDM) any of the carrier frequencies canbe used as input (e.g., using an analog or digital filter). If a higherfrequency is used, then the wavelength that determines the value of d issmaller, which may help to facilitate design of a more compact chargingstation.

Some power line signals operate in 2-30 MHz band. However, signals canbe generated at lower power levels at higher frequencies (e.g., up to 80MHz). Use of higher frequencies would facilitate smaller tap spacing,thus reducing the overall size of the charging station. At 80 MHz, forexample, a ¼ wavelength tap spacing would be about 24 inches. Smallerspacings of 12 inches or even less would be effective as well.Permissible signal levels at 80 MHz are lower than in the 2-30 MHzrange, but given the short range of the charging connection and theabsence of other impairments in the charging connection (e.g.,transformers or fuse panels), these power levels may be adequate.

All or portions of the directional awareness system 200 other than thepower line segment and the taps may be implemented with any combinationof analog and/or digital circuitry. Additionally, alternative systemscan provide the same phase shifts provided by the transmission lines222, 224, 226, and 228 digitally after receiving signals from Tap A andTap B directly and converting the analog signals into digital signals.The system 200 has the advantage that it is simple and cheap to buildand can provide reliable directional awareness even for small tapseparation distances that are considerably shorter that the longestwavelength components of the propagating signal, allowing for a compactcharging station device.

In some implementations directional awareness is provided withoutnecessarily providing full or partial directional communication usingPLC signals. In some implementations, circuitry for receiving and/ortransmitting PLC signals over the power line segment can use one or moreof the taps used in the directional awareness system. For example, thesignal from the first adder 232 or the second adder 234 can be providedto receiving circuitry to demodulate and decode a signal, as describedin more detail below. Additionally, a signal can be transmitted over thepower line segment using circuitry (not shown) coupled to one or more ofthe taps.

A directional awareness system may also be implemented with adirectional coupler as disclosed in U.S. Application Ser. No.61/043,581, incorporated herein by reference. The directional couplerprovides both directional awareness and communication, and enables astation to receive PLC signals propagating in a selected direction onthe power line segment, and also enables a station to transmit PLCsignals in a selected direction on the power line segment. Thedirectional coupler is described in more detail below.

Referring to FIG. 3, a directional coupler 300 includes a firsttransceiver 302 and a second transceiver 304 coupled to a transmissionline 306 (e.g., a power line) through respective taps. The tap 308 (or“tap A”) for the first transceiver 302 and the tap 310 (or “tap B”) forthe second transceiver 304 are coupled to the transmission line 306separated by a physical distance “d” along the power line ofapproximately ¼ of a wavelength at the center of the frequency band usedfor modulating signals. Equivalently, this separation distance is thedistance at which a sinusoidal signal propagating from one tap to theother undergoes a phase shift due to propagation of 90 degrees. Forexample, for a PLC system operating at 30-50 MHz, the separation wouldbe approximately 1.25 meters, assuming a propagation velocity on thepower line of ⅔ the speed of light in a vacuum. Other systems mayoperate over other frequency ranges as high as 80 MHz, for example. At80 MHz, a quarter wavelength tap spacing could be achieved at tapspacings as small as 0.63 meters. Tap spacings well under a quarterwavelength will still enable determination of the direction of signalpropagation based on the relative gain caused by interference, or basedon direct measurements of the phase shift at one tap compared to thephase shift at the other tap due to propagation of the sinusoidal signalfrom one tap to the other. The reliability of the determination ofdirection of propagation may be enhanced by sampling the relative phaseshift between Tap A and Tap B for multiple corresponding subcarriers.Small phase shifts due to small tap spacings are difficult to measure inthe presence of high noise levels (low SNR). However, if manysubcarriers are sampled (e.g. one hundred), errors due to random noisewill average out, resulting in a highly reliable indication of thedirection of signal propagation. The more subcarriers used in making thedetermination, the higher the confidence in the result. The number ofsubcarriers to use in any given circumstance depends on the frequenciesused, the confidence required in the result, the SNR, and the tapseparation distance. The spacing of the two taps at a distance ofapproximately a quarter wavelength at the center frequency of the bandis used in some implementations because it enhances forward gain.However, other factors such as physical coupler size may dictate thatthe directional coupler is implemented with a tap spacing other than aquarter wavelength at center frequency, or may dictate making adetermination of the direction of signal flow using only the highestfrequencies in the signal rather than the center frequency, as describedin more detail below.

As described above, in some implementations directional awareness isprovided without necessarily providing full or partial directionalcommunication using PLC signals. For example, in the exemplary coupler300 of FIG. 3, either or both of the first transceiver 302 and secondtransceiver 304 can function only as a receiver and not as a transmitterand still provide the directional awareness described herein.

Any of a variety of modulation schemes may be implemented by thesynchronized transceivers 302 and 304, that convert data to and from asignal waveform that is transmitted over the communication medium (e.g.,the power line 110). One exemplary modulation scheme is OrthogonalFrequency Division Multiplexing (OFDM). To illustrate how thedirectional coupler 300 functions, the operation of the system with anOFDM modulation scheme will be described in detail. First a the OFDMmodulation scheme for signals coupled to and from an individual tap willbe described to explain OFDM concepts, and then additional techniquesfor processing signals for the multi-tap directional coupler will bedescribed.

In OFDM modulation generally, data are transmitted in the form of OFDM“symbols.” Each symbol has a predetermined time duration or symbol timeT_(s). Each symbol includes a Guard Interval (to combat the effects ofmultipath distortion) and a Fast Fourier Transform (FFT) evaluationperiod (T_(FFT)). OFDM symbols are generated from a superposition of Nsinusoidal waveforms that are orthogonal to each other over the periodT_(FFT) and form the OFDM subcarriers. Each subcarrier has a peakfrequency ƒ_(i) and a phase Φ_(i) measured from the beginning of thesymbol. For each of these mutually orthogonal subcarriers, a wholenumber of periods of the sinusoidal waveform is contained within thesymbol time T_(FFT). Equivalently, each subcarrier frequency is anintegral multiple of a frequency interval Δƒ=1/T_(FFT). The phases Φ_(i)and amplitudes A_(i) of the subcarrier waveforms can be independentlyselected (according to an appropriate modulation scheme) withoutaffecting the orthogonality of the resulting modulated waveforms. Thesubcarriers occupy a frequency range between frequencies ƒ₁ and ƒ_(N)referred to as the OFDM bandwidth. The direction of signal propagationcan be determined using only a portion of the subcarriers (e.g., apredetermined number of subcarriers having the highest carrierfrequencies), or even a single subcarrier. It would therefore bepossible to determine the direction of signal propagation by analyzingonly the subcarriers located at the highest frequencies in thecommunications channel, thereby enabling implementation of a anapparatus with much smaller tap spacing.

For example, to further enhance reliability, the direction of signalpropagation can be determined by analyzing relative phase shifts betweenmany corresponding subcarriers at the first tap (308) and second tap(310) based on observations made on several OFDM symbols. Using multiplesubcarriers with a given OFDM symbol and/or multiple OFDM symbols (e.g.,a predetermined number of successive OFDM symbols) enhances reliabilitybecause measured phase shifts between corresponding subcarriers providesfrequency and time diversity. Especially when the tap spacing is smallresulting in a small phase shift between carriers received at differenttaps, sources of random errors such as thermal noise and quantizationerrors can overwhelm the small differences in phase shift. However,random errors in phase shift measurements will tend to average to zeroover many measurements while systematic effects such as phase shifts dueto signal propagation are additive.

In many power line communications systems, the signal-to-noise ratio(SNR) of each subcarrier is known by the receiving station a priori. Theeffects of random errors in phase shift measurements can be furtherreduced by weighting phase shift measurements on subcarriers with highSNR more heavily than phase shift measurements on subcarriers with lowSNR. The phase shifts measured for each subcarrier may be multiplied bythe respective subcarrier wavelengths and a weighting factor thatdepends on the SNR for that subcarrier before being added to generate areliable a tap separation estimate with a sign that is a reliableindicator of the direction of propagation. The weighting factors mayalso be determined in part based upon the frequencies of thecorresponding subcarriers, to favor or emphasize the measurements ofhigher frequency subcarriers over lower frequency subcarriers.Subcarrier SNRs may also be used to select a subset of the availablesubcarrier for use in the direction determination.

Referring to FIG. 4, a communication system 400 includes a transmitter402 for transmitting a signal (e.g., a sequence of OFDM symbols) over acommunication medium 404 to a receiver 406. The transmitter 402 andreceiver 406 can both be incorporated into a network interface module ateach station. The communication medium 404 can represent a path from onedevice to another over the power line network.

At the transmitter 402, modules implementing the physical (PHY) layerreceive a data unit from the medium access control (MAC) layer. The dataunit is sent to an encoder module 420 to perform processing such asscrambling, error correction coding and interleaving.

The encoded data is fed into a mapping module 422 that takes groups ofdata bits (e.g., 1, 2, 3, 4, 6, 8, or 10 bits), depending on theconstellation used for the current symbol (e.g., a binary phase shiftkeyed (BPSK), quadrature phase shift keyed (QPSK), 8 point QuadratureAmplitude Modulated (8-QAM), 16-QAM, 64-QAM, 256-QAM, or 1024-QAMconstellation), and maps the data value represented by those bits ontothe corresponding amplitudes of in-phase (I) and quadrature-phase (Q)components of a subcarrier waveform of the current symbol. This resultsin each data value being associated with a corresponding complex numberC_(i)=A_(i) exp(jΦ_(i)) whose real part corresponds to the I componentand whose imaginary part corresponds to the Q component of a subcarrierwith peak frequency ƒ_(i). Alternatively, any appropriate modulationscheme that associates data values to modulated subcarrier waveforms canbe used.

The mapping module 422 also determines which of the subcarrierfrequencies ƒ₁, . . . , ƒ_(N) within the OFDM bandwidth are used by thesystem 400 to transmit information. For example, some subcarriers thatare experiencing fades can be avoided, and no information is transmittedon those subcarriers. Instead, the mapping module 422 uses coherent BPSKmodulated with a binary value from the Pseudo Noise (PN) sequence forthat subcarrier. For some subcarriers (e.g., a subcarrier i=10) thatcorrespond to restricted bands (e.g., an amateur radio band) on a medium404 that may radiate power no energy is transmitted on those subcarriers(e.g., A₁₀=0). The mapping module 422 also determines the type ofmodulation to be used on each of the subcarriers (or “tones”) accordingto a “tone map.” The tone map can be a default tone map, or a customizedtone map determined by the receiving station, as described in moredetail below.

An inverse discrete Fourier transform (IDFT) module 424 performs themodulation of the resulting set of N complex numbers (some of which maybe zero for unused subcarriers) determined by the mapping module 422onto N orthogonal subcarrier waveforms having peak frequencies ƒ₁, . . ., ƒ_(N). The modulated subcarriers are combined by IDFT module 424 toform a discrete time symbol waveform S(n) (for a sampling rate ƒ_(R)),which can be written as

$\begin{matrix}{{S(n)} = {\sum\limits_{i = 1}^{N}{A_{i}{\exp \left\lbrack {j\left( {{2{\pi }\; {n/N}} + \Phi_{i}} \right)} \right\rbrack}}}} & {{Eq}.\mspace{14mu} (1)}\end{matrix}$

where the time index n goes from 1 to N, Ai is the amplitude and Φ_(i)is the phase of the subcarrier with peak frequency ƒ_(i)=(i/N) ƒ_(R),and j=√−1. In some implementations, the discrete Fourier transformcorresponds to a fast Fourier transform (FFT) in which N is a power of2.

A post-processing module 426 combines a sequence of consecutive(potentially overlapping) symbols into a “symbol set” that can betransmitted as a continuous block over the communication medium 404. Thepost-processing module 426 prepends a preamble to the symbol set thatcan be used for automatic gain control (AGC) and symbol timingsynchronization. To mitigate intersymbol and intercarrier interference(e.g., due to imperfections in the system 400 and/or the communicationmedium 404) the post-processing module 426 can extend each symbol with acyclic prefix, or Guard Interval, that is a copy of the last part of thesymbol. The post-processing module 426 can also perform other functionssuch as applying a pulse shaping window to subsets of symbols within thesymbol set (e.g., using a raised cosine window or other type of pulseshaping window) and overlapping the symbol subsets.

An Analog Front End (AFE) module 428 couples an analog signal containinga continuous-time (e.g., low-pass filtered) version of the symbol set tothe communication medium 404. The effect of the transmission of thecontinuous-time version of the waveform S(t) over the communicationmedium 404 can be represented by convolution with a function g(τ; t)representing an impulse response of transmission over the communicationmedium. The communication medium 404 may add noise n(t), which may berandom noise and/or narrowband noise emitted by a jammer.

At the receiver 406, modules implementing the PHY layer receive a signalfrom the communication medium 404 and generate an data unit for the MAClayer. An AFE module 430 operates in conjunction with an Automatic GainControl (AGC) module 432 and a time synchronization module 434 toprovide sampled signal data and timing information to a discrete Fouriertransform (DFT) module 436.

After removing the cyclic prefix, the receiver 406 feeds the sampleddiscrete-time symbols into DFT module 436 to extract the sequence of Ncomplex numbers representing the encoded data values (by performing anN-point DFT). Demodulator/Decoder module 438 maps the complex numbersonto the corresponding bit sequences and performs the appropriatedecoding of the bits (including deinterleaving and descrambling).

Referring back to FIG. 3, the directional coupler 300 implements a PHYlayer modulation scheme, in this example OFDM, with the added featuresuppressing the signal transmitted or received in one direction ofpropagation on the transmission line. The two transceivers 302 and 304are controlled by a common signal processing unit 312. The signalprocessing unit 312 enables the signals transmitted and received by thetransceivers 302 and 304 to be processed using common signal references,such as a common clock reference, which can be used to define a commonphase reference for setting relative phases between the subcarriers, orsubset of subcarriers used at the charging station for determining thedirection of signal propagation, at the two transceivers. In someimplementations, the signal processing unit 312 generates basebandsignal characteristics such as amplitudes and phases to be used formodulating respective subcarrier frequencies in a multi-carriermodulation scheme (e.g., OFDM as described in more detail above and inU.S. Publication No. 2006/0256881 A1 and U.S. Application No.60/941,949, each of which is incorporated herein by reference). Fortransmission, at tap A, an information bearing signal 322, designatedINFO, is coupled onto the transmission line 306 by the first transceiver302. At tap B, a second signal 324, designated CANCEL, with apredetermined relationship to the INFO signal is coupled onto thetransmission line 306 by the second transceiver 304. For reception, theINFO and CANCEL signals are used to couple a signal propagating in aselected direction from the transmission line 306.

The purpose of the CANCEL signal 324 is to steer a directional null byinterfering with the INFO signal 322 in one (e.g., REVERSE) directionwhile receiving a signal arriving from another (e.g., FORWARD)direction. Because of the predetermined physical distance between thesignal taps and the predetermined phase relationships betweensubcarriers emanating from the signal taps, as described in more detailbelow, the two signals combine constructively in the desired (FORWARD)direction—thus providing gain in the FORWARD direction. For the purposeof illustration, assume that there are no major signal impairments orimpedance mismatches in either the FORWARD or REVERSE directions. Inthis idealized situation, the CANCEL signal uses the same subcarrieramplitudes as the INFO signal with a predetermined phase shift (or“rotation”) of each subcarrier of the OFDM signal such that the twosignals nullify each other in the REVERSE direction. The exact phaserotation for each subcarrier is dependent on the distance, d, betweenthe two signal taps, A 308 and B 310, and the frequency of thesubcarrier.

The predetermined relative phase shift between subcarriers at tap A 308and tap B 310 are established explicitly or implicitly based on a commonphase reference at tap A and tap B. The transceivers 302 and 304 couplesignals to and from the taps according to a common phase reference thatis provided, for example, by the signal processing unit 312, orestablished by some technique for establishing a common phase reference(e.g., using synchronized clocks as described in U.S. Publication No.2007/0025398 A1, incorporated herein by reference). For signalreception, dynamically controlled phase shifts between subcarriersgenerated at tap A and tap B, relative to this common phase reference,enable dynamically selectable cancellation in the REVERSE direction andconstructive interference in the FORWARD direction. By switching thedirection of FORWARD and REVERSE and comparing the resulting receivedsignal strength, the (previously unknown) direction of an arrivingsignal propagating on the transmission line 306 can be determined.

An imposed relative phase shift β(λ) of a subcarrier of wavelength λwithin the CANCEL signal 324 relative to the same subcarrier in the INFOsignal 322 can be determined as follows. The phase shift Φ(λ) acquireddue to signal propagation of the subcarrier over physical distance “d”is:

Φ(λ)=2πd/λ  Eq. (2)

where λ is the wavelength of the subcarrier on the transmission line306. Thus, the acquired phase shift is proportional to the distancebetween the taps divided by the wavelength of the subcarrier on thetransmission line, or equivalently, proportional to the distance betweenthe taps multiplied by frequency of the subcarrier where the frequencyƒ=ν/λ (where ν is the propagation speed on the transmission line).(Typically, a subcarrier has a spectrum with a spectral shape that has amaximum value (or “peak”) at a “peak frequency” and tapers off away fromthe peak frequency. For example, in OFDM modulation the subcarriers havea spectral shape that is approximately a sinc function. The wavelengthor frequency of the subcarrier corresponds to the wavelength orfrequency at a peak of the spectrum.) In the REVERSE direction, when theCANCEL subcarrier signal 324 propagating from tap B 310 (shown as thedashed line) reaches tap A 308, it is combined with the INFO subcarriersignal 322 propagating from tap A (shown as the solid line). Theresulting combined signal will be cancelled if the sum of the imposedrelative phase β(λ) and the acquired relative phase shift Φ(λ) add to π(or 180 degrees). The imposed relative phase shift β(λ) for a givensubcarrier (having a wavelength λ) in the CANCEL signal at Tap B iscomputed as:

β(λ)=π−Φ(λ)   Eq. (3)

In this case, the amplitude of the subcarriers in the CANCEL signal areidentical to those in the INFO signal.

In the FORWARD direction, the INFO subcarrier signal 322 (shown as thesolid line) starts propagating from tap A 308 with a phase shift of−β(λ) relative to the same subcarrier in the CANCEL signal 324. When theINFO subcarrier signal reaches tap B 310, it is combined with the CANCELsignal propagating from tap B (shown as the dashed line) according to atotal phase that is the sum of the initial relative phase shift −β(λ)and the acquired relative phase shift Φ(λ), which adds to Φ(λ)−β(λ). Forthe subcarrier for which the distance d is λ/4, the acquired relativephase shift Φ(λ) is π/2 and the imposed relative phase shift β(λ) isπ−π/2=π/2. Thus, for this subcarrier, the INFO subcarrier signal iscombined with the CANCEL subcarrier signal according to a total relativephase shift of zero, resulting in a doubling of the signal amplitude.For subcarriers for which λ/4 is less than or greater than the distanced, the INFO and CANCEL subcarrier signals are added with a nonzerorelative phase 4πd/λ−π, but still add constructively over a relativelylarge wavelength range.

The designation of which transceiver provides the “INFO” signals andwhich provides the “CANCEL” signals is arbitrary, such that the choiceof direction in which signals are canceled and direction in whichsignals constructively add can be controlled dynamically depending onthe desired direction of reception or transmission of a signal. Tocontrol the direction, the directional coupler 300 applies theappropriate relative phase shift by imposing a phase shift on either orboth of the taps. For example, to switch directions, the INFO and CANCELsignals can each be applied to different taps such that the imposedphase shift β(λ) is applied to the signal at tap A 308 instead of thesignal at tap B 310. Equivalently, to switch directions, the INFO andCANCEL signals can be applied to the same taps with the sign of theimposed phase shift β(λ) changed such that an imposed phase shift of−β(λ) is applied to the signal at tap B. The direction in which a signalis cancelled will be called the REVERSE direction, but this directionmay be dynamically controlled to be either direction on the transmissionline (i.e., LEFT or

RIGHT). It is also possible to linearly combine two different signalssimultaneously in order to “listen” in both the FORWARD and REVERSEdirections on the transmission line at the same time (e.g., if arecharging station is configured to receive signals from both a localvehicle and the authentication server at the same time).

Due to reciprocity, the null steering described above describes thebehavior of the directional coupler 300 for transmitting signals (alsocalled “transmit mode”) and for receiving signals (also called “receivemode”). The two signal taps A 308 and B 310 can be thought of as alinear array. An array factor that represents a gain for couplingsignals between the transmission line 306 and an array of taps appliesto both transmission gain and reception gain. A null (or at least apartial null—e.g., reduction of 20 dB or 50 dB or more) in the arrayfactor can be provided in one direction on the transmission line while auseful signal level or even a gain greater than 0 dB or as high as 3 dB,for example, is provided in the other direction. The same imposed phaseshifts as calculated above can be used to process signals received atthe two taps to recover signals from one direction while blockingsignals from the other direction. The signals received at each tap areprocessed to impose the required phase shifts using a programmable phaserotator on each subcarrier and the two resulting signals may be addedtogether prior to demodulation to suppress and substantially cancelsignals that are propagating on the transmission line in a one directionwhile passing or enhancing signals that are propagating the otherdirection on the transmission line. This enables the directional couplerto simultaneously receive two different signals from opposite directionson the transmission line.

Other implementations of the directional coupler 300 are possible. Forexample, in some implementations, a single transceiver can providesignals to and receive signals from both taps. In other implementations,one tap is coupled to a transceiver while the other tap is coupled to areceiver. This configuration still enables the directional coupler toreceive a signal and determine the direction of propagation of thatsignal in receive mode when it is not necessary to impose directionalsteering in the transmit mode.

In some implementations of the directional coupler, any number of taps(e.g., three or four or more) can be used to provide a dynamicallycontrollable level of destructive interference to null or nearly nullsignals in one direction and provide non-nulling interference orconstructive interference (gain) in the other direction. For example,more than two taps may be used to increase the effective bandwidth overwhich gain is provided in one direction while nulling the otherdirection. In some implementations, there is only a single carrierinstead of multiple carriers (or “subcarriers”).

Referring to FIG. 5, if the CANCEL signal is adapted to maximize nullingin the REVERSE direction for each subcarrier within the signalbandwidth, signal gain in the FORWARD direction will not be uniformacross the entire signal bandwidth. The INFO and CANCEL signal vectorswill combine perfectly in phase on only one of the subcarrierfrequencies (e.g., the center subcarrier frequency). Subcarriers notlocated at band center will have slightly less FORWARD gain. Thisresults in an amplitude taper across the signal bandwidth in the FORWARDdirection. FIG. 5 is a plot of signal gain in the forward direction,measured in decibels, as a function of frequency for system that doesnot use amplitude prescaling. The center subcarrier at 40 MHzexperiences a forward gain of approximately 6 dB (3 dB of gain is due tothe array factor and 3 dB is due to the fact that twice as much power isbeing injected into the system relative to a system using a singletransceiver). Subcarriers at other frequencies closer to the edges ofthe signal bandwidth experience reduced forward signal gain due toamplitude taper.

This amplitude taper is generally undesirable because regulatory limitsare imposed on transmitted power. If the signal at band center is heldwithin regulatory limits, the signal at band edge will be furthersuppressed due to the aforementioned amplitude taper.

Referring to FIG. 6, the INFO signal may be prescaled in amplitude tocompensate for this amplitude taper across the signal bandwidth. In thiscase, amplitude prescaling is accomplished by inverting the gain curvein FIG. 5 and normalizing gain at band center to 0 dB. When theprescaled signal is injected into the directional coupler, the result isan ideal flat gain across the entire signal bandwidth as shown in FIG.6.

The fully implemented directional coupler can simultaneously receivesignals from both directions of propagation. Thus, a charging stationusing a full directional coupler may monitor signals on the line fromall PLC nodes and simply ignore or discard billing information receivedin signals propagating from the network toward its charging terminal.

Many other implementations other than those described above are withinthe invention, which is defined by the following claims.

1. An apparatus for coupling a charging station to a power line segmentthat is terminated at a first end by a charging terminal, the apparatuscomprising: multiple taps coupled to the power line segment; andcircuitry coupled to the charging station and coupled to the multipletaps, with the circuitry being configured to differentiate betweencommunication signals propagating on the power line segment in thedirection from the first end to a second end of the power line segmentand communication signals propagating on the power line segment in thedirection from the second end to the first end based at least in part onmultiple measurements of respective phase shifts associated withdifferent portions of a communication signal, each portion received overat least a first tap and a second tap.
 2. The apparatus of claim 1,wherein the communication signal comprises multiple subcarriers withdifferent frequencies, and the respective phase shifts associated withdifferent portions of the communication signal comprise phase shiftsassociated with different subcarriers.
 3. The apparatus of claim 2,wherein each of the multiple measurements of the respective phase shiftscomprises determining a phase for a subcarrier received over the firsttap relative to a phase for the subcarrier with the same frequencyreceived over the second tap.
 4. The apparatus of claim 3, wherein thecommunication signal comprises multiple symbols occurring sequentiallyin time, and the respective phase shifts associated with differentportions of a communication signal comprise at least some phase shiftsassociated with different subcarriers and at least some phase shiftsassociated with different symbols.
 5. The apparatus of claim 1, whereinthe communication signal comprises multiple symbols occurringsequentially in time, and the respective phase shifts associated withdifferent portions of a communication signal comprise phase shiftsassociated with different symbols.
 6. The apparatus of claim 5, whereinthe communication signal comprises multiple subcarriers with differentfrequencies, and each of the multiple measurements of the respectivephase shifts comprises determining a phase for a subcarrier receivedover the first tap relative to a phase for the subcarrier with the samefrequency received over the second tap.
 7. The apparatus of claim 1,wherein the circuitry comprises: a first phase detector connected to thefirst tap; a second phase detector connected to a second tap; and acomparator with inputs connected to an output of the first phasedetector and an output of the second phase detector.
 8. The apparatus ofclaim 1, wherein the circuitry comprises: a first adder connected to thefirst tap via a first transmission line and connected to the second tapvia a second transmission line that is longer than the firsttransmission line; a second adder connected to the second tap via athird transmission line and connected to the first tap via a fourthtransmission line that is longer than the third transmission line; and acomparator with inputs connected to an output of the first adder and anoutput of the second adder.
 9. The apparatus of claim 1, wherein thecharging station is a vehicle charging station that provides power to avehicle connected to the charging terminal.
 10. The apparatus of claim1, wherein the charging station is configured to determine billinginformation from received communication signals propagating on the powerline segment in the direction from the first end to the second end. 11.The apparatus of claim 1, wherein the charging station is configured todetermine billing information from communication signals transmitted bya vehicle connected to the charging terminal.
 12. The apparatus of claim1, wherein a first tap on the power line segment is spaced a distance Dfrom a second tap on the power line segment.
 13. The apparatus of claim12, wherein the distance D is less than about one tenth of thewavelength of at least one of a predetermined number of the highest ofmultiple carrier frequencies of the communication signals.
 14. Theapparatus of claim 13, wherein the distance D is less than about onetenth of the wavelength of the lowest of the predetermined number of thehighest of multiple carrier frequencies of the communication signals.15. The apparatus of claim 1, wherein the circuitry comprises: a signalprocessing unit in communication with each of the taps, wherein thesignal processing unit is configured to measure each phase shift bydetermining a phase for at least a portion of a signal received over atleast a first of the taps relative to a phase for a correspondingportion of the signal received over at least a second of the taps. 16.The apparatus of claim 15, wherein the communication signal comprisesmultiple subcarriers, and the signal processing unit is configured tomeasure each phase shift by determining a phase for each subcarrierreceived over at least a first of the taps relative to a phase for acorresponding subcarrier received over at least a second of the taps.17. The apparatus of claim 16, wherein the corresponding subcarrierreceived over the second of the taps has the same frequency as thesubcarrier received over the first of the taps.
 18. The apparatus ofclaim 16, wherein the multiple subcarriers comprise fewer than allsubcarriers on which the signal is modulated.
 19. The apparatus of claim16, wherein the multiple subcarriers comprise a predetermined number ofsubcarriers with the highest frequencies.
 20. The apparatus of claim 16,wherein the signal processing unit is configured to apply a weight toeach measured phase shift according to a signal-to-noise ratioassociated with the subcarrier for which the phase shift is measured.21. A method for coupling a charging station to a power line segmentthat is terminated at a first end by a charging terminal, the methodcomprising: sensing a communication signal propagating on the power lineand being coupled through multiple taps connected to the power linesegment; and determining if the communication signal is propagating onthe power line segment in the direction from the first end to a secondend of the power line segment or in the direction from the second end tothe first end based at least in part on multiple measurements ofrespective phase shifts associated with different portions of thecommunication signal, each portion received over at least a first tapand a second tap.
 22. The method of claim 21, wherein the communicationsignal comprises multiple subcarriers with different frequencies, andthe respective phase shifts associated with different portions of thecommunication signal comprise phase shifts associated with different subcarriers.
 23. The method of claim 22, wherein each of the multiplemeasurements of the respective phase shifts comprises determining aphase for a subcarrier received over the first tap relative to a phasefor the subcarrier with the same frequency received over the second tap.24. The method of claim 23, wherein the communication signal comprisesmultiple symbols occurring sequentially in time, and the respectivephase shifts associated with different portions of a communicationsignal comprise at least some phase shifts associated with differentsubcarriers and at least some phase shifts associated with differentsymbols.
 25. The method of claim 21, wherein the communication signalcomprises multiple symbols occurring sequentially in time, and therespective phase shifts associated with different portions of acommunication signal comprise phase shifts associated with differentsymbols.
 26. The method of claim 25, wherein the communication signalcomprises multiple subcarriers with different frequencies, and each ofthe multiple measurements of the respective phase shifts comprisesdetermining a phase for a subcarrier received over the first taprelative to a phase for the subcarrier with the same frequency receivedover the second tap.
 27. The method of claim 21, wherein the chargingstation is a vehicle charging station that provides power to a vehicleconnected to the charging terminal.
 28. The method of claim 21, whereinthe communication signal includes billing information transmitted from avehicle connected to the charging terminal.
 29. The method of claim 21,wherein the second end is connected to a power distribution network. 30.The method of claim 29, wherein the communication signal includesbilling information transmitted from a vehicle connected to the chargingterminal.
 31. The method of claim 29, wherein the communication signalincludes billing information transmitted from a server connected thepower distribution network.